Could Your Antihistamine Be Aging Your Brain?

Antihistamine medication is commonly used to treat allergies such as hay fever, conjunctivitis, rhinitis, eczema, and hives, as well as reactions to insect bites.

They are also used as sedatives in some psychiatric units.

With many antihistamines available over the counter in tablet, cream, lotion, or spray form, the side effects of this seemingly harmless type of drug are rarely mentioned.

But could your allergy medication be harming your brain?

Understanding antihistamines

Histamine is a chemical produced by the immune system as part of the body’s defence mechanism to fight infection.

Whenever there is a foreign body like a virus or bacteria detected by the immune system, histamine is released into the cells nearby, causing inflammation.

Allergic reactions essentially ‘fool’ the immune system into perceiving certain harmless foreign bodies (like pollen, dust, or some food ingredients) as a threat to be repelled. Histamine is released and again causes inflammation and irritation.

Antihistamine medication works to change the behaviour of the cells, blocking the receptors and preventing the inflammation from occurring.

While antihistamine medication has advanced over the years, it is still associated with certain side effects, including:

• Drowsiness (in the case of ‘first generation’ antihistamines)
• Reduced reaction times
• Difficulty passing urine
• Dry mouth
• Blurred vision
• Headache
• Nausea
• Dependency

These side effects may be worse if you have underlying medical conditions or combine the medication with alcohol or antidepressants. Their toxic effect on pregnant women is also a major concern.

With many people taking antihistamines daily for long periods or even for the rest of their lives, we need to be aware of the potential dangers that antihistamines pose.

They are not necessarily as safe as they first seem…

The Alzheimer’s question

Antihistamines have been linked with causing dementia.

A US study published over 20 years ago found that long-term or high-dose use of antihistamine-based hay fever and depression medication (including drugs available over the counter) was linked to a higher risk of dementia.

In the study, the author recommends increasing awareness amongst health professionals and older adults about the potential risks.

Another study from around the same time, published in the journal Medical Hypotheses, refers to studies of the effects of antihistamine medication on schizophrenics.

Benadryl and other antihistamines are frequently used in psychiatric facilities as sedatives. The study found a possible connection between these antihistamines and Alzheimer’s disease:

“Several large studies of schizophrenics have revealed an extremely high prevalence of Alzheimer's disease neuropathology compared with that in the general population. The neuroleptic treatment of schizophrenia appears to be implicated in this phenomenon. Many of the neuroleptics have anticholinergic effects, some being antihistamines as well, including chlorpromazine. It is proposed here that anticholinergics, including many antihistamines, either exacerbate the Alzheimer process or possibly contribute to its etiology/pathogenesis through their effects on cerebral cholinergic systems.”

The author includes a recommendation for “parsimony” in the use of antihistamines “pending the resolution of this issue.”

But has this issue actually been resolved yet - over two decades later?

Benefit vs. cost

Allergy sufferers should weigh up the benefits of their medication against the potential side effects and other risks before committing to take antihistamines.

They are not always effective and rarely eliminate symptoms totally.

And, despite being one of the most used medications in the UK, the potential dangers associated with taking antihistamines are significant but seldom discussed. They certainly warrant more investigation to resolve the issue.

References:

Lione A. and Scialli A.R. The Developmental Toxicity of the h1 histamine antagonists. Reproductive Toxicology, Vol. 10, No. 4, pp. 247-255, 1996

Oken R.J. Antihistamines, a Possible Risk Factor for Alzheimer's Disease Medical Hypotheses (1995) 44, 47-48

NHS website. http://www.nhs.uk/conditions/Antihistamines/Pages/Introduction.aspx. Accessed 13th Sept 2016.

Gray S.L et al. Cumulative Use of Strong Anticholinergics and Incident Dementia. JAMA Intern Med. 2015;175(3):401-407. doi:10.1001/jamainternmed.2014.7663